After I completed my deck, I was proud of it and, of course, wanted to show it off to all my friends. What started as a small gathering ended up pretty large, and there were a lot of people on my deck. Afterward, I began to wonder how much weight can a deck hold?
If your deck is built to code, then it is designed to hold a minimum of 50 pounds per square foot, of which ⅘ of that total is the “live” load. Therefore, a deck of 120 square feet multiplied by 50 shows that your deck can hold 6000 pounds, including the structure. Dividing by 5 will allow you to determine that 4500 lbs is the max live load your deck can hold.
There are many, many factors that affect how much weight your deck can hold. Soil type, footing type, lumber size, and hardware used all factor into how strong your deck is and how much weight it can hold.
Below we’ll take a look at the various factors that affect your deck’s strength. As well, we’ll show you resources you can use to ensure you are building your deck strong enough. Lastly, we’ll take a look at the different types of lumber you can use for your deck and how it will affect the weight capacity of the overall structure.
- How Much Weight Can a Deck Support?
- Calculating Deck Weight Capacity
- Deck Load Variables
- How to Determine if a Deck Can Support a Hot Tub
- Deck Weight Limits for Composite and Other Types of Decking
How Much Weight Can a Deck Support?
If your deck was built to code and the appropriate permits were pulled and signed off on, then it is entirely likely your deck holds a minimum of 50 pounds per square foot.
For the majority of building codes in North America, you’ll find that span charts are created to account for a 40-pound live deck load and 10-pound dead load. That means that the overall structure can support a load of people, or “stuff”, of at least 40 pounds per square foot on top of your deck.
Let’s take a 12×10 foot deck as an example. At the beginning of the article, we determined that the total live load the deck could sustain was 4500 pounds, after subtracting for the “dead” load, which is the deck structure itself.
Now keep in mind that the deck is meant to sustain that load over its entirety – not in a concentrated area. Therefore if you have a hot tub that weighs 4500 pounds, that doesn’t mean your deck can support it since that weight is only in a concentrated area. If you want to actually put people in that hot tub, you’ll need to reinforce your deck.
If you are not sure if your deck is built to code, then fortunately you can still determine how much weight your deck can hold. However, you’ll have to get underneath — or at least have a good view of the underside of your deck – to see the dimension of lumber it was built with as well as to measure the distance between footings, footing type, and the type of hardware used.
Calculating Deck Weight Capacity
Before you calculate how much weight your deck can hold, you first need to understand how much weight per square foot it can hold. Most decks will hold more – sometimes significantly more – than 50 pounds per square foot. That number is simply a minimum requirement.
The average homeowner does not have the tools available to determine how much more weight your deck can hold than the code minimum of 50 pounds per square foot. However, you can determine if your deck is overbuilt, which can give you an idea of how much more weight your deck can hold.
One of the best tools in your arsenal to determine deck weight capacity are deck load calculators and max span calculators, widely available online. If you find that your deck joists are 2×10, but the deck load calculator only mandates 2×8 joists, then you can safely assume your deck is much stronger than 50 pounds per square foot.
Deck Load Examples
Let’s take our 12×10 example and investigate the weight capacities of different types of beams. For our purposes, we’ll assume our deck is 12 feet long and 10 feet wide. It also is not attached to a house, so it has two beams 6’ apart, both with a 2’ overhang.
If you go under this deck and find you have 3 ply 2×8 beams, then you can use a span table to determine footing placement. You’ll also have to know the type of lumber used – in this case it is SPF since we are up north.
A 3 ply 2×8 beam for a deck with a width of 10’ allows for a maximum span of 6’. In the case of our deck, we have footings every 4’. I can now safely assume that my deck can hold much more than 50 pounds per square foot, as indicated by the deck span chart.
Tributary Deck Loads
The amount of weight that each individual footing can hold is called the tributary load. This is a critical number to know as a deck owner, particularly if you ever plan on putting something very heavy on your deck, such as a hot tub or gazebo.
Calculating tributary loads is not difficult, as long as you have a diagram of your deck’s framing structure. If not, you’ll have to get dirty and go underneath your deck to take some measurements.
Calculating a tributary load is fairly simple if you have a standard rectangular deck attached to your house. You’ll measure the distance from the ledger board to the center of the support beam and draw a line. Let’s say that distance is 14’. That means half of the deck closest to the house is entirely supported by the ledger board.
The other half is supported by the footings supporting the beam. Next, you measure the distance between the footings. Divide that distance in half. Draw a rectangle from that point up to the first line halfway between the house and beam. Do that for each midway point between footings. Now you have an area for each footing.
Now it is simply a matter of finding the area for each rectangle. Multiply each area by 50, and you have the number of pounds per square foot that each footing can hold.
Deck Load Variables
There are many variables when it comes to calculating deck load capacity. Below I’ve listed a few of the major areas that give a deck the ability to sustain large amounts of weight. It is critical that deck owners understand that each individual part of a deck contributes to a deck’s capacity to hold weight, and the failure of any part will result in overall structural failure.
Curious if your deck’s footings are up to code? Then check out this footing load table, which can tell you exactly how big your footings need to be based on the size of the post, the span of the beam, and the length of your joists.
Above, we calculated the tributary load of a deck, which tells us how much weight each footing can sustain. Knowing the tributary load of each footing can help us determine the diameter of each footing. Once we find out the appropriate diameter, we can then know if our deck is overbuilt and, thus, capable of sustaining more weight.
First, you divide the tributary load of 1500 – the weight-bearing capacity of the default soil type used to determine building codes. We’ll say the tributary load of your interior deck footing is 3000 pounds. 3000/1500 is 2. This number is the footing area – but we need the diameter.
Let’s change that area of 2 square feet into inches. One square foot has 144 square inches – so for us it’s 288 square inches.
Remember that pi x diameter equals a circle’s area. Reverse that and divide your area by pi. 288/3.14 = ~96. Then find the square root, and multiply times 2. We get just over 19” for our footing diameter. If your footing is 24” sonotube, then it is more than strong enough to support your deck.
But as mentioned above – an overbuilt footing still depends on a properly constructed deck frame. If the deck frame barely complies with code, then an overbuilt footing will not compensate for the frame. It will simply ensure that the deck does not sink into the ground.
Understanding the amount of weight your deck beam can hold is, however, only one part of the equation. Knowing your soil type is just as important.
Just as 50 pounds per square foot is the minimum weight capacity for a deck built to code, so too is soil bearing capacity. But what does that mean? It means different types of soil vary in terms of how much weight they can support before whatever they are supporting sinks into the earth.
That being said, most weight-bearing calculators for decks assume a default of 1500 pounds per square foot for soils. This is for soils that are a mix of loam, silt, and clay, which also happens to be one of the weaker types of soil.
If your deck is built on bedrock, gravel, or even sand, then you’ll have a higher – and potentially even a much higher – soil bearing capacity. For instance, a deck built on bedrock will have a bearing capacity of nearly 12,000 pounds per square foot.
That doesn’t mean that your deck will suddenly have the same bearing capacity – it only means that your deck will not sink – ever. On the other hand, if you have soil that is extremely wet, all clay, or simply unstable, then it is likely your soil will not sustain the minimum of 1500 pounds as dictated by building code.
If that is the case, you could have the strongest deck frame in the world, but bad soil will cause your deck to sink under loads that the frame of your deck is designed to support. Soil matters.
The type of lumber used is also critical in determining how much weight your deck can hold. The three major types of wood species used in deck construction are Southern Yellow Pine (SYP), some combination of Spruce, Pine, and Fir (SPF), and Redwood, Western Cedar, or Red Pine.
If you look at the joist span tables above, it doesn’t take long to see that SYP is stronger than the other lumber species. It is only slightly stronger than SPF lumber or Redwood and Wesetern Cedar.
For instance, a 3-ply 2×12 beam can span 8’ between footings when joists are 12’ long when both SPF or Western Cedar is used. SYP lumber can span 10’ – 2’ more than the other species.
How does this affect load-bearing capacity? Again, knowing your deck lumber species can allow you to see if your deck is overbuilt.
Using the example above, if you have a beam of that size built with SYP lumber, and you have a span of 10’ but with three footings instead of two, you can safely assume the overall weight-bearing capacity of the deck is far more than 50 pounds per square foot.
One of our last variables in determining deck weight bearing capacity, fasteners, are arguably the most important aspect of deck construction and the most overlooked.
A key indication of your fastener load bearing capacity is what you’ve used to affix beams to joists and posts to beams. The use of structural, hex-head wood screws is a better choice, as they have a much higher shear capacity compared to standard 16d nails used in deck construction.
However, if you are using post to beam connectors, joist hangers, and hurricane ties to support your structure, then standard 10d nails are just fine – the hardware itself will provide most of the support. The key is to make sure you use the appropriate nails or structural screws, as indicated by that piece of hardware.
Have a look underneath your deck. Take a close look at the hardware. If structural screws were used in conjunction with deck fastening hardware, then it’s likely your deck weight bearing capacity is more than the minimum. If only nails were used, without hardware of any type, then you might want to think twice before placing an 8000-pound hot tub on your deck.
Deck Snow Loads and Weight Capacity
The snow load of a deck is factored into the 50 pounds per square foot standard found in so many North American building codes for decks. Why? Simply because it is assumed that you will clear your deck prior to inviting 20 of your friends over for a beer on that deck.
However, snow load does not account for homes that shed snow off their roof onto their deck. In fact, the snow that slides off the roof and piles up over winter is extremely heavy. It also risks collapsing a deck. Therefore, if you have a deck beneath a roof eave where snow will slide onto, you either need to regularly clear your deck or install a roof snow guard of some type.
Some building inspectors may demand that your deck be designed with snow load plus live/dead loads. In that case, you’ll need to know what the average snow load is per your geographic region – a snow load chart helps.
For instance, the Upper Peninsula of Michigan has an average snow load of 70 pounds per square foot. Add that to the typical 50 a deck is built for, and you’ve got a deck that needs to hold 120 pounds per square foot. That means more footings and/or larger dimensional lumber, 12” o.c. construction, and less joist overhang.
While accounting for snow load in the U.P. is probably wise, you’ll notice that North Carolina, for instance, only has an average snow load of 15 pounds per square foot. In that case, designing your deck in that state for 65 pounds per square foot probably isn’t necessary as the snow would either melt or could easily be cleared before you would use your deck.
How to Determine if a Deck Can Support a Hot Tub
Putting a hot tub on a deck requires understanding the tributary loads of each part of your deck. Luckily, we now know how to calculate those loads since we did it a few paragraphs above.
Can you put a hot tub on a deck? Absolutely, but we’ve got to know how much weight each of our footings can handle. Since a hot tub will more than likely fall into the area of just one, or maybe two, footings, we need to know that one footing can handle the weight of a hot tub.
Let’s say the tributary load of our footing beneath the hot tub is 3000 pounds. But our hot tub is 3900 pounds – what do we do? We’ll have to install another footing beneath the beam. Or, we can place the hot tub where two or three tributary load areas intersect. That way, the weight is divided amongst three footings.
Deck Weight Limits for Composite and Other Types of Decking
Trex claims it supports 100 pounds per square foot, although that number is not based on any specified criteria for a deck structure. And it does not really matter because the frame of a Trex deck is what supports the deck, not the deck boards; therefore, a Trex deck is no stronger than a lumber deck.
For cedar decks, check out this table:
Using cedar lumber for the framing of your deck is perfectly acceptable but way more expensive than common pressure-treated lumber. It is also not as strong, which will result in potentially more footings since cedar cannot span the same distances as pressure-treated wood such as SPF or SYP.
However, a properly constructed cedar deck to specs of 50 pounds per square foot will be able to hold the same amount of weight as a deck that has used typical pressure-treated wood.
Remember, you don’t need to be a structural engineer to determine how much weight your deck can handle. Using the resources above, you can find out both your deck’s total weight capacity and the deck load capacity for each footing on your deck.
But keep in mind those are just estimates. It would be difficult to verify every last detail of your deck structure to ensure it is up to code, particularly the thickness of concrete footings and soil type.
If you want to host a large party on your deck but still aren’t sure if it can handle the weight, it’s best to contact a building engineer. It may cost you, but you’ll know for sure the limitations of your deck and the steps for rectifying any issues.
As always, thanks for taking the time to read this article. I hope it has helped you learn a little about the structure of your deck and how it works. Please feel free to drop me a line and let me know how I can improve this article for future readers.